In today's society, batteries are widely used in various devices including portable electronic products such as notebooks, cameras, cellular phones, MP3 or the like, as well as cars, robots, satellites or the like. The batteries can be classified into primary and secondary batteries and among these, secondary batteries are extensively used due to their great advantages of being able to repeatedly charged/discharged and store energy.
Currently, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries or the like are commercially available as secondary batteries. Among these, lithium secondary batteries are at the center of interest because they hardly have memory effects and can be freely charged or discharged in comparison with nickel-based secondary batteries. Advantageously, the lithium secondary batteries also exhibit very low self-discharge rate and high energy density.
Such secondary batteries may be used in the form of one battery cell or multiple battery cells connected in series in a battery system. However, the latter case makes it possible to output and store even higher power. In particular, with the gradual exhaustion of carbon energies and the increased interest in the environment recently, the use of a battery system using a secondary battery in large-sized equipments is increasing. Since the battery system used in large-sized equipments requires high output, multiple battery cells connected in series are used.
Exemplary large-sized battery system having multiple battery cells connected in series may include a smart grid system.
The smart grid system is an intelligent electric grid system which incorporates an information and communication technology into the production, transfer and consumption process of power, thereby inducing the interaction of power supply and consumption and thus enhancing the efficiency of electricity usage. The amount of electric energy used by a consumer may not always be constant and can frequently change. For example, in the summer time, the electric energy consumption can increase rapidly due to the use of an air conditioner in the afternoon and rapidly decrease later on in the evening. Like above, the amount of electric energy consumed may not always be constant and can frequently change, and even if the power supply is controlled to some degree, it is realistically impossible to meet the varying electric energy consumption amount. Accordingly, the imbalance of the power supply and electric energy consumption may cause an excess or deficiency of the power supply. In order to solve such a problem, a smart grid system checks for the power status in real time to flexibly control the power supply. A battery system for storing power is an important component in constructing such a smart grid system. Such a battery system for storing power has much more battery cells connected in series than those of a battery system used in typical electronic devices or portable electronic products.
A battery system using multiple battery cells connected in series is useful not only in a smart grid system, but also in a micro grid system and other various fields such as an electric vehicle charging station for supplying power. Moreover, although not as much as the smart grid system, micro grid system, and electric vehicle charging station, the battery system using multiple battery cells connected in series is useful in a battery pack for hybrid or electric vehicles requiring high power output so as to obtain the driving force of vehicles.
However, the battery system using multiple battery cells connected in series may be subject to performance degradation in some battery cells. That is, a situation can occur where some of the battery cells included in the battery system may not operate normally even though most of the battery cells therein are operating normally. Conventionally, although most of the battery cells are still useful, such a performance degradation of some of the battery cells may deteriorate the overall performance of the battery system which may interrupt the use of the battery system itself. Also, in some cases, in spite of the presence of multiple battery cells operating normally, the battery system itself may be exchanged due to its characteristics that the battery cells are connected in series, which may waste unnecessary time and costs.